Design of a three-dimensional gap-coupled suspended substrate stripline bandpass filter

A three-dimensional gap-coupled suspended substrate stripline (SSS) bandpass filter, based on the modified configuration of the classic end-coupled filter, is presented. It consists of printed half-wavelength resonators placed on the same and/or the opposite sides of the suspended substrate. These resonators orient themselves in arbitrary directions. Such an arrangement makes the new filter highly flexible to interface with other microwave subcircuit modules. The filter is constructed in a bent channelized housing that suppresses any possibility of exciting higher-order modes, except the dominant quasi-TEM mode of propagation. Consequently, the channelized filter is free from any coupling between itself and other microwave circuit components within the system. Two look-up tables, derived from a variational three-dimensional quasi-TEM spectral-domain analysis of the discontinuity problems associated with the adjacent gap-coupled resonators, are incorporated into the computer-aided design of the new filter. The de-embedded discontinuity parameters are validated by comparing them with those obtained by experiment and full-wave approaches for the microstrip open-end and the SSS end-coupled problems, respectively. The measured results for the experimental 21-22 GHz bandpass filter agree very well with the theoretic predictions considering the conductor losses (5.57×10^-3 dB/mm) of the SSS. It has less than 1.4 dB insertion loss and greater than 10 dB return loss in the passband.